Don't do SASL LIST MECH

The server just supports PLAIN anyway

Change-Id: I62fe5caf05bde9d9eb21a340eda04f6eea5d523d
Reviewed-on: http://review.couchbase.org/24246
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>